Three-phase commercial chemical processes that include a gas, a liquid, and a solid catalyst are typically carried out in slurry reactors or packed bed reactors. In a slurry reactor, a solid catalyst is suspended by some means of agitation in a reaction vessel containing the gas and a relatively large volume of liquid. In a packed bed reactor, a solid catalyst is stationary within a vessel under the pressure of a gas, and a liquid is supplied to the catalyst in forms such as a trickle or film.
Typically, mass transfer processes in the reaction vessel limit rates of reaction; thus, promotion of mass transfer is an important factor in choosing a reactor for commercial processes. In addition, due to safety concerns and energy costs, the ability for a reactor to dissipate heat is also important. Other factors to consider in choosing a reactor are manipulation of the catalyst, maintenance of the reactor, and recovery of the often expensive catalyst. For a more detailed discussion of the common chemical reactors see A. Gianetto and P. L. Silveston, Multiphase Chemical Reactors: Theory, Design, Scale-Up, Hemisphere, N.Y., 1986.
Slurry reactors are often used in chemical industrial processes because they have superior mass transfer attributes. Some commercial three-phase processes that employ slurry reactors are the hydrogenation of unsaturated fats and the oxidation of alkenes. Typically, there is a much greater amount of liquid than solid in a slurry reactor, and the agitation to suspend the catalyst results in high velocity flow of the liquid. Therefore slurry reactors have excellent mass transfer attributes and relatively high dissipation efficiency.
The immersion of heat exchangers in slurry reactors is usually not problematic, and this further increases their advantages in heat dissipation. However, one problem with slurry reactors is that there is usually a relatively small amount of catalyst present in the reactor. This can lead to decreased efficiency compared to processes requiring no agitation.
Another disadvantage of such reactors comes in the catalyst recovery stage, since the finely divided particles are often difficult to filter on a large scale. Additionally, if the catalyst is pyrophoric, the filtration recovery phase can raise serious hazard concerns. In general, the above mentioned problems with slurry reactors can make large scale-ups a challenging endeavor.
Packed bed reactors are also common for three-phase commercial processes, examples of such reactions including hydroprocessing, hydrogenation, and oxidation processes. The most common type of packed bed reactor employs a tightly packed catalyst bed onto which a liquid is introduced as a trickle and flows downward by gravity. A reactant gas is also fed co-currently and its flow is driven by pressure drop.
Since liquid being processed in such reactors is introduced into a stationary bed of catalyst and no agitation is required, the reactor can provide a relatively high volume ratio of catalyst to liquid feedstock.
One problem with packed bed reactors is that they must be packed and maintained with high uniformity to avoid the formation of channels through the bed. Another disadvantage to packed bed reactors is that the beds can become plugged when small diameter particles are used. This problem can be mitigated by the use of larger particles, but this strategy will also reduce the efficiency of the bed. As with slurry reactors, the problems mentioned above with packed bed reactors can make scale-up difficult.
Monolithic catalyst reactors for three-phase chemical reactions offer a number of advantages over slurry and packed bed reactors while avoiding some of the disadvantages. In monolithic catalysts of honeycomb type, the catalyst is formed of, dispersed within, or manufactured as monolithic honeycomb structures comprising a plurality of parallel channels formed by intersecting channel walls running from one end of the structure to the other. The channels can have any of a number of cross-sectional shapes, including triangles squares, rectangles, and hexagons, and channel sizes over a relative broad range of channel diameters ranges from fractions of a millimeter to centimeters or more.
Monolithic catalysts are commonly used in the treatment of automotive exhaust gases, and their use in three-phase catalytic reactions has been the focus of recent studies. Included among the latter is the monograph xe2x80x9cThe Use of Monolithic Catalysts for Three-Phase Reactionsxe2x80x9d by S. Irandoust, A. Cybulski, and J. A. Moulijn in Structured Catalysts and Reactors, edited by A. Cybulski and J. Moulijn, Marcel, Dekker, New York 1998.
In a monolithic catalytic reactor employed in a three-phase reaction, a feed stream comprising a combination of reactant gases and reactant liquids is passed through the channels of the honeycomb. Feed stream flow may be in a co-current (gas and liquid flowing in the same direction) or counter-current (gas and liquid flowing in opposite directions) flow mode.
For certain velocities and volumetric ratios of the gas to the reactant liquid, and particularly in co-current flow modes, the flow of the mixture is segmented as a sequence of gas bubbles and liquid plugs of similar size, with an absence or only small quantity of gas in the reactant liquid plugs. Segmented flow of this type is said to be in the Taylor or Taylor flow regime.
Taylor flow is presently considered to be a desirable flow mode for honeycomb monolith reactors because it provides a relatively thin layer of reactant solution against the channel walls past which the gas bubbles are conveyed. In addition, Taylor flow provides good recirculation within the liquid plugs. The thin layer and good recirculation promotes mass transfer in a monolithic catalyst reactor that can approach that of a slurry or packed bed reactor. When the segmented flow reaches the end of the reactor, it can be collected and the gas and liquid separated to both collect the product and recycle the gas.
The potential advantages of honeycomb monolith catalyst reactors over slurry and packed bed reactors are several. First, since the channels of the monolith are relatively open, there is only a small pressure drop across the entire reactor, and the drop can be two orders of magnitude lower than that of packed bed reactors. Further, monolithic reactors can resist the bed plugging seen in packed bed reactors and do not require any form of agitation. Thirdly, the monolith reactor can be easily scaled-up, because the scale-up implies merely adding a greater number of monolithic catalyst segments and adjusting the inlet flows. And finally, recovery of the catalyst is generally significantly easier than that of slurry reactors, since there is no filtration required.
Some potential problems of monolithic catalyst reactors include relatively poor heat dissipation and difficulty initiating and maintaining acceptable Taylor flow. For example, monoliths are presently manufactured only in relatively small sizes, such that honeycomb reactor packings may require the horizontal joining and/or vertical stacking of several catalyst sections. Particularly at horizontal junctions between vertically arranged catalyst layers, flow disturbances can disrupt the flow of gases and liquids through the individual honeycomb channels. This can make maintaining an acceptable Taylor flow difficult.
An additional problem is that, as the segmented flow proceeds through the reactor, the gas streams or bubbles can be depleted of gas due to reaction, which may move the stoichiometric and volumetric gas-to-liquid (G:L) ratios out of the ideal value to maintain Taylor or other desired flow modes. Prior art three-phase monolithic catalytic processes have no effective means for replenishing reactant gases to balance G:L ratios and maintain desired flow modes.
The present invention is generally directed to the use of multiple stages of gas injection along the length of a monolithic catalyst reactor composed of monolithic catalyst beds in series. The multi-staged injection maintains a desired volumetric gas to liquid volumetric ratio (G:L) (defined at the temperature and pressure of interest) in the monolith channels in order to maintain the segmented flow near or at the Taylor regime. The staged injection concept of the invention maintains the desired G:L locally in the monolith bed, while managing the overall stoichiometry of the reaction. The global G:L set by stoichiometry may be different than the desired local value.
The present inventive process is a general one that can be applied to many different three-phase reactions, examples of which include hydrogenation of olefins, dienes, styrenes, aromatics, and the reduction of partially oxidized species such as aldehydes to alcohols. The process is not limited to these reactions, but can be applied to any catalytic hydrogenation or hydrotreating process or process that utilizes a reactive gas and a feedstock liquid. Other reactions where the invention can provide improved efficiency in monolithic catalyst beds include nitration, amination, sulfonation, chlorination, sulfidation, cyanation, and fluorination. The flow management advantages of the invention extend to upward or downward co-current flow modes, and to certain counter-current flow modes as well.
The invention further encompasses a reactor for maintaining desired values of the G:L in a three-phase monolithic catalyst reactor, in order to maintain a segmented gas/liquid flow near or at a Taylor or other desirable flow regime. Since maintenance of proper segmented flow is most sensitive to the gas bubble component, the current invention maintains proper G:L ratios during a process by multi-staged injection of reactant gas into the bubble components of the segmented gas/liquid flow.
These and other features of the present invention are more fully set forth in the following description of illustrative embodiments of the invention.